In the ever present desire to reduce the size and weight of components, especially those that are in mobile applications, such as a vehicle for instance, the reduction of component size and weight while maintaining or improving overall component functionality is highly desirable. To effectuate this component size and weight reduction while at least maintaining component functionality, the power density of the component needs to increase, requiring advanced materials, construction techniques, and electrical power management.
In the case of an electrical motor as an example, increasing power density would typically entail increasing the motor rotational speed—basically to increase motor power output while reducing motor torque levels resulting in a smaller and lighter weight motor.
However, increasing motor speed creates a number of design issues to overcome that primarily occur in two areas, being the motor itself and electrical control of the motor. For the motor itself, the design issues would include structural integrity of the motor rotor to withstand high rotational speeds, motor rotor shaft bearings again to withstand high rotational speeds, clearances as between static and dynamic parts, and temperature rise considerations due to friction and electrical inefficiencies which become predominate at higher power densities, as smaller motor size equates to lower surface areas and lower volumes to absorb and shed heat buildup. A conventional lower rotational speed motor rotor construction (typically termed squirrel cage induction), utilizes a complex multi piece built-up rotor with iron plates and copper wire windings-being limited to a maximum rotational speed of about twenty thousand revolutions per minute (20,000 rpm) due to rotational centrifugal stress and excessive heat buildup in the iron and copper due to electrical inefficiency.
A high speed motor rotor is constructed of a permanent magnet which requires a structural reinforcement member to allow the permanent magnet to withstand high rotational speeds of around one hundred thousand revolutions per minute (100,000 rpm) or even higher rpm. Although the permanent magnet reduces the previously mentioned electrical inefficiency of the squirrel cage induction rotor, the addition of the structural reinforcement that is positioned between the high speed motor rotor and stator increases electrical inefficiency as the structural reinforcement acts as an electrical and physical barrier between the rotor permanent magnet and the rotating electromagnetic field of the stator. Although the structural reinforcement member is non-magnetic—it is constructed of a metallic material for the needed strength.
Another set of issues need to be dealt with in high speed motors are rotor speed sensing and rotor position sensing both in relation to the motor stator, both of which are needed for more optimum electrical control. On a conventional low speed motor, typically a Hall effect sensor or rotary encoder would be used for instance utilizing a rotor physical notch that passes by an electromagnetic field sensor every rotor revolution resulting in the electromagnetic field being periodically interrupted to thus give an indication of rotor speed and position. The Hall effect sensor works well, however, having drawbacks of size and structure needed to accommodate the sensor in proximity to the motor rotor plus the need for an asymmetrical discontinuity in the rotor to give the sensor a “read” section for rotor speed and position. The requirements of the sensor and asymmetrical discontinuity are generally not desirable in the high speed motor for several reasons; the sensor structure consumes physical space which on a proportional scale is problematic in a smaller high speed motor, plus in addition, the asymmetrical discontinuity in the rotor also consumes physical space and further interferes with achieving optimal rotor balance and causes added stress in the rotor, wherein desirable design in a high speed motor would dictate minimal physical structure space consumed and minimal rotor size that is completely symmetric in design.
Thus, in a high speed motor, eliminating the sensor and eliminating the asymmetrical discontinuity in the rotor are both highly desirable, however, there remains the need for knowing rotor position and speed for the motor controller to work in a more optimum manner, so these items of rotor position and rotor speed must be determined in another manner without a physical sensor and rotor asymmetrical discontinuity. The result would be a “sensorless” way to determine rotor position and rotor speed—i.e. meaning that the high speed motor has none or minimal added components within the high speed motor itself—so as not to interfere with optimal high speed motor design, which leads to a method and structure external to the high speed motor to determine rotor position and speed electronically as a unique way to more optimally control a high speed motor, which incidentally would typically be of a smaller size to accommodate very high rotor speeds as previously discussed.
One sensorless high speed motor solution is to use motor back electromotive force (EMF) as a way to determine motor rotor speed and rotor position while the motor is operational—i.e. the rotor is rotating, however, having the problems that no back EMF is generated when the motor rotor is stopped, which can lead to rotor position error at motor startup, which could cause reverse rotor rotation. Another solution is to embed in the stator winding a way of measuring stator winding electrical activity to derive rotor position and rotor speed. In both of these scenarios, either the EMF signal or the stator winding electrical activity signal must be significantly processed to produce meaningful output based upon test data.
High-speed turbo machines, such as turbochargers, or air blowers have been or are being developed to take advantage of new areas of technology enabled by these permanent magnet brushless direct current motors termed BLDC motors, also accompanying advances in microcontrollers, micro control units termed MCUs, field programmable gate arrays termed FPGAs, and power switching transistors circuits, have enabled significant advances in motor controllers. Thus, these high-speed permanent magnet brushless direct current motors are characterized by the desire for high power density, a wide speed range, and high acceleration rates that go considerably beyond conventional electric motor capabilities. The potential for the high power density of a permanent magnet brushless direct current motor that is coupled with the desire for reduced production costs, leads to a magnetic topology that requires proper current control to minimize motor, and to some extent control system, losses.
The most practical and lowest cost control for these permanent magnet brushless direct current motors is a three phase alternating current with a six switch inverter that is operated with block commutation. As a phase is energized the current rises to a set point and pulse width modulation or as termed PWM is employed to provide the calculated energy to a phase for a calculated duration of time based on motor demand. The PWM frequency can be chosen for a variety of reasons, however, in high-speed motor control, the highest practical switch speed has to be utilized to achieve very high frequencies, further PWM is used to provide desirable symmetrical sine wave forms to the motor. In contrast to this requirement, several loss mechanisms with the controller and motor are tied to the frequency content of the current waveforms, with losses tending to rise with increasing frequency or increased motor rotational speed. This results in a trade-off between high losses and usable motor speed. Limitations on the preferred switch type, being preferably an insulated gate bipolar transistor or as termed IGBT are typically capable of about 20 to 25 kHz maximum or are potentially limited by the maximum allowable losses in the system design, other switching transistors could be metal oxide semiconductor field effect transistors or as termed MOSFET or related equivalents. The problem with this is that it results in high switching losses and limits upon the motor rotational speed achievable.
High-speed motor design for the permanent magnet brushless direct current motor typically results in a two pole machine with a slug or ring magnet utilized. These particular motors have distributed windings that can yield trapezoidal or sinusoidal back electromotive forces termed EMF, wherein the motor EMF is a function of motor shaft rotational speed. Current day high-energy magnet materials include Neodymium Iron Boron or Samarium Cobalt which being somewhat similar to a ceramic material in a strength of materials nature, are reasonably tolerant of compressive stress and intolerant of tensile stress. For this reason, the magnet is often retained with an interference fit metal band or a cylindrical “can” over the magnets thus keeping the magnets in a compressive prestressed state such that the magnets do not experience tensile stress associated with high levels of centrifugal force due to high motor rotational shaft speeds. This of course brings in a complication as this metal “can” or could be termed a structural reinforcement sleeve needs to have fairly high strength in a tensile manner and also have high electrical resistivity to minimize eddy current losses during BLDC motor operation.
The structural metal magnet reinforcement sleeve requires a more sinusoidal current waveform or a waveform with reduced harmonic content, and motor airgap flux density profile, in order to prevent rotor eddy current losses. Controllers are designed for these motors using sinusoidal current waveforms and sine wave filters to provide the desired sinusoidal current waveform or reduced harmonic content waveform. The sine wave controller with prior art sine wave filters is typically large, expensive, and inefficient with a limited optimum speed range. Utilizing simple block commutation as previously described with a high-speed metal structural sleeve banded motor rotor results in current waveforms and subsequent motor air gap flux profiles that cause excessive rotor heating and thus motor inefficiency.
In looking at the prior art in this area in U.S. Pat. No. 6,424,798 to Kitamine, disclosed is a sensorless brushless-DC-motor mounted on an electric or hybrid vehicle that is powered by an on-board battery through an inverter supplying a three-phase pulse width modulated voltage (PWM voltage). In Kitamine, the inverter is controlled to generate the PWM voltage having an average voltage level corresponding to a target motor speed. The PWM voltage level in Kitamine is controlled by changing its duty ratio, so that a difference between the target motor speed and an actual motor speed is minimized. The actual motor speed in Kitamine is determined based on a signal indicating a rotor position detected from the PWM voltage imposed on the motor, wherein the rotor position circuit (element 13) is shown in FIG. 2, with the circuit eliminating a carrier frequency from the PWM voltage to determine a rotor position through voltage differential between phase voltage and average motor voltage, with a speed detector (element 14) which calculates motor speed as a reciprocal of a cyclic period of pulse signals indicating rotor position. When the battery voltage drops in Kitamine and the duty ratio becomes 100%, the target motor speed is temporarily reduced to the level of the actual motor speed, further when the battery voltage is recovered and the duty ratio becomes lower than 100%, the target motor speed is gradually increased again to the original level. In this manner in Kitamine, even if the battery voltage abruptly increases, the PWM voltage is properly controlled, and thereby the motor is stably driven without causing loss of synchronism.
Continuing in the prior art, in U.S. Pat. No. 5,345,156 to Moreira, disclosed is a method and apparatus for operating a brushless permanent magnetic motor at speeds where zero crossings of stator phase voltages becomes unavailable, wherein applications of stator currents to one stator phase is temporarily halted to allow current in the phase to reach zero, and the zero crossings of the internal motor voltage is detected as a surrogate for the zero crossings of the stator phase back EMF. Thus a controller in Moreira can react to the detected zero crossings as if the phase current was being applied, in using a processes third harmonic component of the stator voltage in conjunction with back EMF of a single phase for motor control.
Next, in the prior art in U.S. Pat. No. 8,138,694 to Steigerwald, et al., disclosed is a bidirectional buck-boost power converter 13 including a pair of inverter modules 14, 15 disposed at an output of a machine, and an inductor L connected between the pair of inverter modules 14, 15. Thus in Steigerwald, a method for controlling a voltage output of a machine (turbine) starter generator having an inverter rectifier and bidirectional buck-boost converter, includes outputting a DC voltage controlled by bidirectional buck-boost pulse width modulation (PWM) switching control, when the starter generator is in a generator mode.
Further, in the prior art in U.S. Pat. No. 8,080,960 to Rozman, et al., disclosed is a flux regulated permanent magnet brushless motor with a stator having an inner peripheral bore. In Rozman, a permanent magnet rotor is mounted within the inner peripheral bore and a control winding is supplied to a DC current to regulate flux of the machine with a small AC current also being supplied, wherein an output is sensed to determine a position of the permanent magnet rotor.
Next in the prior art in U.S. Pat. No. 7,230,361 to Hirzel, disclosed is an electric device, such as an electric motor, a generator, or a regenerative motor, having a wound stator core made from advanced low-loss material. In preferred embodiments in Hirzel, the electric device is an axial airgap-type configuration wherein the invention provides an electric device having a high pole count that operates at high commutating frequencies, with high efficiency and high torque and power densities. Thus in Hirzel, advanced low-loss materials exploited by the present invention include amorphous metals, nanocrystalline metals, and optimized Fe-based alloys.
What is needed is a low cost, compact, and efficient controller that has a more sinusoidal current waveform or a waveform with reduced harmonic content, to operate the low cost type permanent magnet brushless direct current motor. Thus, the solution that is disclosed is a block commutation type controller that when operated at the proper direct current source voltage has desirable attributes to control the low-cost BLDC motor type. The direct current supply voltage, when operated to match the motor operating back EMF, or slightly above the motor operating back EMF, results in a more sinusoidal waveform or a lower harmonic content waveform, and in current control mode the switching transistors no longer have PWM and transition to full on or off mode. Wherein a usable controller-motor results in being able to shift the design point dynamically in relation to motor rotor speed, as opposed to a prior art controller that utilizes a set input voltage resulting in a single motor rotor speed design point. Thus, in other words, the maximum IGBT switching rate and all the other rate related losses are reduced significantly due to the elimination of PWM when the voltage used to feed the direct current controller is at a level just slightly in excess of the peak voltage produced by the motor/generator at the desired operating speed. The ideal excess voltage is determined by the sum of the voltage drops for the controller, the BLDC motor/generator, and all associated wiring at the current level required to maintain the desired speed.
The difficulty with maintaining this condition is that the required voltage changes with every change to the desired speed of the motor output shaft and further with a shaft load at that speed, thus being a very dynamic condition. In addition, this operating mode will dramatically limit the available power at a low motor shaft output speed due to the linear relationship between speed and output voltage. Thus, a structure and method is disclosed that provides numerous advantages for the operation of very high rotational speed BLDC motor/generators.
Further as an alternative option to using back EMF is to embed in the motor stator winding a wire loop for measuring stator winding electrical activity to derive rotor position and rotor speed through a processed signal from the wire loop. In both of these scenarios, either the EMF signal or the stator winding electrical activity signal must be significantly processed to produce meaningful output based upon test data and further an optional look up table of known values can augment the EMF signal or the stator winding electrical activity signal for increasing accuracy in determining motor rotor speed and position in a sensorless manner.